The present invention relates to a method for producing a structural steel plate for a welded steel pipe for conveying a fluid containing wet hydrogen sulfide such, for example, as natural gas or crude oil containing H.sub.2 S, or for a tank for storing such fluid, and provides a method for producing a steel plate having an excellent resistance to cracking due to wet hydrogen sulfide, commonly referred to as hydrogen induced cracking.
Welded steel pipes and steel tanks have been used for conveying or storing natural gas or crude oil. With the recent progress in development of fields producing crude oil and natural gas having relatively high contents of hydron sulfides, there has been an increasing demand for steel pipes and tanks which are highly resistant to hydrogen induced cracking, for conveying such crude oil or natural gas. Particularly, large-diameter steel pipes for conveying, usually referred to as line pipes, are required to be as low in cost and as high in quality as possible because a vast amount of the pipes are needed for construction of the conveying pipe lines. In recent years the requirement has become increasingly severe. In fact, steel plates that are prevented from cracking under a severe condition such as low PH (&lt;5.0) are demanded in some cases.
A steel pipe for line pipe is presently produced from a hot rolled steel plate manufactured from a continuously cast steel material (or a large steel ingot) by shaping and welding the plate. Particularly in a continuously cast slab, however, there is caused in the course of solidification a segregation generally referred to as a central segregation in the slab thickness center in which the concentration of impurities such as C, Mn, P, S and the like is high, and the central segregation remains in the neighborhood of the center of thickness of the rolled steel plate to increase susceptibility to hydrogen induced cracking of that portion.
Although many attempts were made to control the steel making conditions and to develop new steel making methods, none of them was successful in substantially solving the abovementioned problem. That is, such attempts were made to prevent hydrogen induced cracking as, (I) addition of alloying elements (such as Cu and the like) to thereby inhibit penetration of hydrogen, (II) reduction of S content or proper addition of rare earth elements (REM) or Ca (for spheroidization of inclusions) to thereby reduce the number of possible cracking points, and (III) control of rolling and heat treating conditions to thereby improve the steel structure, but none of them has proved entirely satisfactory. At present, it is considered technically difficult to completely prevent hydrogen induced cracking of continuously cast steel or any other steel material in a severe environment such as low PH condition.